1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to an apparatus for and a method of automatically heating foods in a microwave oven.
2. Description of the Prior Art
As shown in FIG. 1, a microwave oven of the general type having an automatic heating function comprises a microprocessor 1 for controlling the microwave oven, a power supply unit 2 for supplying electric power to the microwave oven, and a magnetron 3 for generating electronic waves by the electric power supplied from the power supply unit 2. In the microwave oven, a heating chamber 4 is provided for receiving the electronic waves from the magnetron 3 and heating a food by the received electronic waves. The heating chamber 4 has an air inlet port 4A and an air outlet port 4B. The microwave oven further comprises a fan 5 for introducing air in the heating chamber 4A through the air inlet port 4A, a pair of temperature sensors 6A and 6B respectively disposed around the air inlet port 4A and the air outlet port 4B and adapted to sense the temperature of air introduced in the heating chamber 4 and the temperature of air exhausted from the heating chamber 4, and a pair of A/D converter units 7A and 7B for converting the temperatures sensed by the temperature sensors 6A and 6B into digital signals and then sending them to the microprocessor 1, respectively.
Now, operation of the microwave oven having an automatic heating function will be described, in conjunction with FIGS. 2 to 4.
When a cooking start button is pressed by a user under a condition that food to be cooked has been put in the heating chamber 4, a select signal corresponding to the cooking start button is sent to the microprocessor 1 so as to start automatic cooking of the food.
Based on the select signal, the microprocessor 1 generates a control signal by which the fan 5 is, in turn, driven for a predetermined time t1 to introduce air in the heating chamber 4 via the air inlet port 4A.
When a temperature equilibrium is obtained in the heating chamber 4 by the lapse of the predetermined time t1, respective temperatures V1 and U1 around the air outlet port 4B and the air inlet port 4A are detected by the temperature sensors 6B and 6A. Based on the detected temperature U1 around the air inlet port 4A, a temperature increment .DELTA.TA in the heating chamber 4 is determined.
In other words, the temperature increment .DELTA.TA in the heating chamber 4 is determined by subtracting the detected temperature U1 from a predetermined reference temperature R to derive an error E between the temperature U1 and the reference temperature R, dividing the derived error E by a general constant F experimentally derived, multiplying the resultant value by a temperature increment .DELTA.T to derive a value .delta., and then adding the temperature increment .DELTA.T to the value .delta..
The temperature increment .DELTA.TA can be expressed by the following equation: EQU .DELTA.TA=.DELTA.T+.DELTA.T(R-U1)/F
wherein, .DELTA.T represents a temperature increment in the heating chamber 4 under a condition that no error occurs.
The temperature increment .DELTA.TA determined in the above-mentioned manner is sent to and then stored in the microprocessor 1.
Thereafter, the magnetron 3 is driven in accordance with a control signal generated from the microprocessor 1 so as to primarily heat the food contained in the heating chamber 4.
During the heating operation, the temperature detected around the air outlet port 4B by the temperature sensor 6B is increased to a temperature V2. When a variation in the air outlet port temperature reaches the temperature increment .DELTA.TA, that is, when the difference between the current temperature V2 and the initial temperature V1 is higher than the temperature increment .DELTA.TA (V2-V1&gt;.DELTA.TA), an additional heating time t3 is then determined by multiplying a time t2 taken for the current temperature V2 of the air outlet port 4A to reach ambient temperature by a predetermined weight a. Thereafter, the food is heated for the determined additional heating time t3.
In other words, the magnetron 3 and the fan 5 are driven for the determined additional heating time t3 to heat the food. After the additional heating time t3 has elapsed, the magnetron 3 and the fan 5 are stopped to complete the heating of the food.
As mentioned above, the temperature increment .DELTA.TA is determined, based on the error E occurring by the temperature U1 of air introduced in the heating chamber 4 through the air inlet port 4A. Accordingly, the temperature increment .DELTA.TA is constant, irrespective of the season.
The additional food heating time t3 is irrespective of the load, namely, the amount of food, because it is determined by multiplying the time t2 taken for the temperature variation in the heating chamber 4 to reach the determined temperature increment .DELTA.TA by the predetermined weight a. In other words, where the amount of food is large, the heating operation is stopped before the cooking of food is completed. As a result, the food may be underdone. Where the amount of food is small, the food may be overdone.
Referring to FIG. 4, there are shown variations in parameters determining the food heating time, that is, variations in temperature increment .DELTA.TA1, .DELTA.TA2, the temperature of the air inlet port 4A and the temperature of the air outlet port 4B. As apparent from FIG. 4, the ambient temperature is approximately constant irrespective of the heating of food. However, the temperature of the air inlet port 4A is gently increased during the heating of food and then sharply increased due to the remaining heat of the magnetron 3 when the heating of food is completed. On the other hand, the temperature of the air outlet port 4B is sharply increased simultaneously with the heating of food and sharply decreased simultaneously with the completion of the heating of food.
Where foods are continuously cooked in the conventional microwave oven having the above-mentioned construction, an execution of a cooking under a condition that the heating chamber has been heated due to an immediately prior cooking causes the magnetron not to operate for a certain time. This causes a problem in that the temperature increment in the heating chamber for cooking is inaccurately calculated because the temperature of the air inlet port is sharply varied during the stoppage period of the magnetron, for example, corresponding to about 3 minutes after the magnetron is stopped while the temperature of the air outlet port is approximately constant during the same period.